To measure blood flow, a hand held probe is typically used to transmit a beam of ultrasonic energy through body tissue to a target blood vessel. Blood cells flowing through the blood vessel scatter the ultrasonic energy in many directions. A portion of the transmitted ultrasonic energy is reflected back to the probe, which receives and processes the reflected energy. In accordance with the well known Doppler phenomenon, the frequency of the received signal is different than that of the source signal due to the velocity (magnitude and direction) of the blood cells. Movement toward the probe compresses the wavelength of the reflected wave, causing an increase in the frequency. Movement away from the probe lengthens the wavelength of the reflected wave, resulting in a decrease in the frequency. This difference between the emitted and received frequencies is known as the Doppler shift. Thus, the speed and direction of blood flow within a blood vessel can be measured in a noninvasive manner using ultrasound emissions and the measured shift in frequency of the received signal. Similarly, a heartbeat, such as a fetal heartbeat, can be measured using ultrasound emissions.
With a continuous-wave (CW) Doppler ultrasound probe, a piezoelectric crystal or element contained inside the probe tip continuously transmits an ultrasonic beam that is reflected by the circulating red blood cells. A separate crystal in the tip continuously receives the reflected sound waves. The transmit and receive crystals are often made from a circular element that has been cut down the middle into two semi-circle shaped elements. The two semi-circles are fixed side by side inside the probe tip with a slight angle to each other to form an intersection of the beam patterns in a patient. Alternate arrangements include using two side-by-side square crystals or a central disk surrounded by an annular ring element. Processing is done on the received signal to extract the Doppler shift frequency. Simplicity of design, ease of use, and low power consumption make CW Doppler the typical choice for small battery powered applications. Also, sensitivity of CW Doppler is typically high because damping of the crystals is not required as known to those skilled in the art
The useful operating frequency range for Doppler ultrasound probes is typically 2-10 megahertz (MHz). The required depth of penetration in body tissue determines the operating frequency based on well-known attenuation effects as a function of frequency. A lower probe frequency provides deeper penetration of the body tissue. Thus, in the medical field, probes having frequencies from about 2 to about 3 MHz may be used to detect deep blood flow, fetal blood flow, or intracranial blood flow due to their deeper penetration of body tissue. Probes having frequencies from about 4 to about 5 MHz may be used to detect vascular blood flow, for example, in the neck, arms, or legs. Probes having frequencies from about 8 to about 10 MHz may be used to detect blood flow in vessels near the skin or in intraoperative applications.
The transmitting piezoelectric crystal is electrically stimulated to produce an ultrasound signal at a specific frequency, for example 2, 3, 4, 5, 8, etc. MHz. The crystal has geometrical and material characteristics that define a specific resonant frequency. CW crystals are typically used undamped with a narrow bandwidth and high Q factor. Operating the undamped crystal at its resonant frequency creates the most efficient ultrasound transmitter and requires the lowest energy power source. Conversely, an undamped receiving crystal is most efficient at producing a voltage when deformed by pressure at or near its resonant frequency. An efficient receive crystal reduces ultrasound exposure risks by allowing lower ultrasound energy to be transmitted into tissue. To change the operating frequency during use, for example from 2 to 3 MHz or from 5 to 8 MHz, a CW ultrasound probe is typically replaced with a probe designed for the desired frequency. Alternatively, the probe can be designed with damped or backed crystals to provide a wider bandwidth of operation and multiple frequencies, but with reduced efficiency due to the wider bandwidth. Additional crystals can be mounted in the probe. For example, two 5 MHZ and two 8 MHz crystals can be mounted in the probe tip. However, the resulting increase in the size of the probe tip make it potentially awkward for a practitioner to use. Thus, a practitioner must carry and manually switch between multiple probes, accept use of a probe having a reduced sensitivity and high transmit power, or use a bulky probe including multiple crystals to provide blood flow measurements at multiple frequencies. What is needed therefore is a system that provides multiple frequencies selectable for optimal signal acquisition in a single probe without reduced sensitivity or loss of Doppler signal. What is further needed is a system that provides the multiple frequencies with little or no increase in the size of the probe tip.